The invention relates to depth gauges for skin divers or frogmen, or those engaged in underwater sports or other activities.
Prior art pressure sensitive instruments in the field to which the present invention relates include devices having a manometer unit provided with a dial on which is marked a graduated scale and over which is movable an indicator needle connected to a pressure-sensitive element which causes displacement of the needle along the scale as the pressure acting on it varies, a hermetically closed capsule which houses the manometer unit and which has a transparent wall covering the dial and the needle, at least one flexible wall, and a liquid which completely fills the capsule and surrounds the manometer unit, this liquid serving to transmit to the pressure sensitive element of the manometer unit the ambient pressure outside the capsule through the said flexible wall.
Known depth gauge instruments of the above mentioned type are called oil-filled depth gauges, since the liquid which fills the capsule is usually a mineral oil.
In order to obtain an accurate indication of depth it is desirable that the needle of a depth gauge is exactly on the zero of the scale at sea level or at the surface of the stretch of water in which the user is going to dive. Oil-filled depth gauges of the type described above are generally calibrated, upon manufacture, in such a way that the needle is at zero at sea level in the so-called normal conditions of atmospheric pressure (760 mm Hg) and temperature (20.degree. C.). As will be appreciated, however, these conditions rarely exist in reality so that the depth gauge would not in general be accurately at zero before a dive.
The variation of atmospheric pressure due to meteorological phenomena does not have a very great effect on the position of the needle, but it is not entirely negligible. Variations in atmospheric pressure with altitude, however, do have a significant effect as a result of which a depth gauge calibrated quite correctly for immersion in the sea will give an incorrect reading if used for immersion in a mountain lake.
Variations of pressure with altitude can be compensated for in depth gauges made at an altitude greater than sea level, by calibrating it in such a way that the needle, at the locality where the depth gauge is made, indicates a value slightly less than zero so that at sea level the reading is at zero in the socalled normal conditions of temperature and pressure. Again, however, the zero setting is entirely accurate only at the one altitude for which it is designed.
More importantly, variations in temperature have a very significant influence on the reading of an oil-filled depth gauge because the oil or other liquid contained in the capsule expands or contracts with changes in temperature causing variations in the volume of the liquid which represent quite significant pressure variations. Thus at temperatures greater or less than the normal envisaged, the needle of the instrument indicates a value respectively greater or less than zero. Moreover, it is not possible to calibrate an oil-filled depth gauge before each dive because the needle and the manometer unit are inaccessible, being sealed in the capsule.